When a person wears spectacle lenses, a variety of elements, such as far vision, near vision, and astigmatism are taken into account. These elements also include elements referred to as visual functions. These visual functions are, for example, functions related to strabismus, heterophoria, aniseikonia, and fixation disparity.
Fixation disparity is a phenomenon that when a target is binocularly viewed (visual fixation), an image of the target cannot be formed at the location of the central fovea of the retina in either one or both eyes, but an image is formed at a location slightly deviated from the central fovea. In other words, fixation disparity is a phenomenon that at the time of the visual fixation of a target, the visual line of either one or both eyes is directed to the direction slightly deviated from the direction of the target.
In the case of a person who has normal stereoscopic visual acuity, if the degree of misalignment of the visual line due to fixation disparity is within a Panum's area, images that have been formed each at a location slightly deviated from the central fovea of the right and left retinas can be perceived as one image by means of processing by the brain, referred to as fusion, thereby enabling stereoscopic vision. For this reason, a person who has fixation disparity is not aware of it and it is also difficult for the person to be aware that vision is impaired. Since density of visual cells rapidly decreases at a location even slightly distant from the central fovea, fine visual acuity cannot be acquired at the location. Therefore, the person with fixation disparity whose visual lines of right and left eye are slightly deviated from the central fovea of right and left retinas due to fixation disparity has decreased visual acuity and perceives a blurred image. Even when an image is blurred only to one eye, the image viewed by the other eye is clear when viewed binocularly; therefore, the person is seldom aware of having deteriorated visual acuity. Accordingly, there are many cases in which constriction of the visual field, eyestrain, headache, stiff shoulder, dizziness, etc. develop without subjective symptoms in addition to a decrease in visual acuity which is a symptom of fixation disparity.
A well-known method of examining the presence or absence of fixation disparity and the degree of fixation disparity is described in patent document 1 (specifically, FIG. 5(b) and FIGS. 6 to 8 in particular). The method described in patent document 1 is as follows:
For example, while a visual target (sign 50) for fixing each visual line of both eyes is displayed on the display unit as a common visual target to both right and left eyes, only the upper visual target (sign 43) is presented to the right eye, and only the lower visual target (sign 45) is presented to the left eye. At this time, on the display unit, both visual targets (43, 45) are displayed at locations that are vertically aligned. However, when both visual targets (43, 45) are presented to a subject who has fixation disparity, the subject perceives both visual targets (43, 45) as being out of alignment as shown in FIG. 6. Then, the subject moves the upper visual target (43) by means of the operating portion, thereby moving the two visual targets (44, 45) so that the subject can perceive both the visual targets (43, 45) as being displayed at locations that are aligned. By doing so, the subject can perceive both visual targets (43, 45) as being aligned. On the other hand, on the display unit, or the real space, since both visual targets (43, 45) have been moved by the subject, both the targets are displayed at locations that are out of alignment with each other. The shift amount (i.e., shift amount in the right and left directions) of both visual targets (43, 45) on the aforementioned display unit corresponds to the fixation disparity amount. Accordingly, the right and left direction (horizontal direction) fixation disparity amount can be obtained. In the same manner, by using the left-side visual target (sign 44) presented only to the left eye and the right-side visual target (sign 45) presented only to the right eye, it is possible to obtain a fixation disparity amount in the vertical direction (top and bottom direction).
Generally, to eliminate the fixation disparity, the spectacles which is prescribed the prisms to cancel the fixation disparity is usually worn by the patient. By wearing the spectacles with the prescribed prisms, many of patients who have fixation disparity will be able to see objects without fixation disparity. Their visual lines of both eyes will be turned for correct directions.
Conventionally, in order to detect a prism amount necessary for eliminating fixation disparity, a subject wore a trial lens provided with a prism, and measurement to check the presence or absence of fixation disparity was repeatedly carried out until a trial lens that helps eliminate the fixation disparity was found. Then, the amount of prism and its base direction of the detected trial lens was used as a prism prescription value.
Thus, the amount of prism and its base direction which has been obtained as the result of trial and error by use of a trial lens or the like and which eliminates fixation disparity when the spectacle lens is worn is referred to as an “aligning prism”. The aligning prism is also a prism prescription value finally provided for a spectacle lens.
Back to patent document 1, it does not describe a method of obtaining an aligning prism. Therefore, even if fixation disparity is measured by the electrical means as shown in the method described in patent document 1, after all, an aligning prism will end up being obtained by adopting a method that uses a trial lens as stated above. That is, a subject wears a trial lens provided with a prism, and measurement is repeatedly conducted until a proper trial lens provided with a prism is found so that the subject can perceive both a pair of visual targets (43, 45) or (44, 45), displayed at locations that are aligned on the real-space display unit, as being located at locations that are aligned.
Furthermore, recently, it has been made clear that there is more than a simple relationship between the fixation disparity amount and the aligning prism, and the relationship significantly varies depending on the subject itself. That is, it has been made sure that even if the fixation disparity amount is the same, the prism amount (aligning prism) necessary for eliminating fixation disparity varies among different individuals (refer to FIG. 8 and FIG. 9 of non-patent document 1).